The present invention relates to a vacuum arc coating process for forming a coating layer by deposition on the surface of cutting tools and other objects in order to improve their wear resistance, a vacuum arc coating machine for this process, and a revolving cutting tool excellent in discharging the chips.
Tools and metal molds required to have high-grade wear resistance have been coated with hard substances such as TiN on their surface to improve the wear resistance. In this case, the coating is carried out by the chemical vapor deposition (CVD) process or the physical vapor deposition (PVD) process. A vacuum arc coating process, a type of PVD process, is superior in the bonding quality between the hard coating layer and the substrate because this process cleans the surface of the substrate by sputtering before the deposition is performed. After the surface cleaning, however, this process produces a coating layer with poor surface roughness, because the surface of the substrate is subjected to deposition of molten particles, known as droplets, readily produced at the same time atoms are transformed to plasma by arc discharges.
In order to solve this drawback, the published Japanese patent application Tokukaihei 10-68071 discloses a process in which nitrogen gas is blown onto the surface of the cathode (the evaporating material) of the evaporating source in order to nitride the surface of the cathode so that the generation of the molten particles is reduced. It reports that the obtained coating layer has a surface roughness of 0.2 xcexcm.
In the process shown in the above application, the nitrogen gas is blown onto the cathode surface in a coating chamber that is directly evacuated by a vacuum pump. Consequently, the blown nitrogen gas is instantaneously sucked in by the vacuum pump, so as to prevent the cathode surface from being nitrided sufficiently. If the amount of the blown nitrogen gas is increased, the partial pressure of nitrogen in the vicinity of the substrate increases, reducing the sputtering effect. Therefore, the coating layer cannot obtain sufficient bonding strength. As a result, the generation of molten particles cannot be suppressed effectively, and the obtained coating layer cannot have sufficiently reduced surface roughness.
Another published Japanese patent application, Tokukaihei 9-170073, discloses that a coating layer with a small magnitude of surface roughness can be obtained when the substrate surface is cleaned with a gas such as a mixture of a rare gas and nitrogen. However, this process uses low partial pressure of nitrogen in the subchamber, so that the surface of the arc evaporation source cannot be sufficiently nitrided. Consequently, the generation of the molten particles cannot be suppressed sufficiently.
The present invention is intended to solve the above-mentioned problems by improving the vacuum arc coating process and machine in order to form a coating layer excellent in the bonding strength with the substrate and in surface roughness.
The present invention relates to a vacuum arc coating process in which an arc evaporation source and a substrate are placed in a vacuum chamber to produce arc discharges at the surface of the cathode of the arc evaporation source so that the generated ions clean the substrate and subsequently form a coating layer on the substrate. More specifically, the vacuum arc coating process of the present invention provides the vacuum chamber with a subchamber having a gas-introducing portion, connects the subchamber to the coating chamber of the vacuum chamber, and places the arc evaporation source in the subchamber. At the time of substrate cleaning, a reactive gas, an inert gas, or a mixture of both is introduced into the subchamber from the gas-introducing portion so that the gas pressure in the subchamber is kept higher than that in the vicinity of the substrate.
The present invention also relates to a vacuum arc coating machine that provides the vacuum chamber with a subchamber having a gas-introducing portion, connects the subchamber to the coating chamber of the vacuum chamber, and places an arc evaporation source in the subchamber.
The present invention also relates to a revolving cutting tool having a coating layer of TiN, ZrC, or TiO, or a combination of them at least on the surface of its grooves. The coating layer has a thickness of 0.2 to 20 xcexcm and an average surface roughness, Ra, not more than 0.05 xcexcm.
The present inventors studied a method to reduce the partial pressure of nitrogen in the vicinity of the substrate while only the surface of the cathode of the arc evaporating source is nitrided. The present inventors anticipated that the foregoing object can be fulfilled by the following process: As shown in FIG. 1, a vacuum chamber 1 is provided with a subchamber 3 that is connected to a coating chamber 2 of the vacuum chamber 1. An arc evaporation source 4 is placed in the subchamber 3. The subchamber 3 is provided with a gas-introducing portion 6 to introduce nitrogen gas. Because the subchamber 3 is less evacuatable than the coating chamber 2, only the partial pressure of the nitrogen in the vicinity of the cathode of the arc evaporation source 4 can be increased.
This arrangement increases the partial pressure of the nitrogen in the subchamber 3 even when a small amount of nitrogen gas is introduced from the gas-introducing portion 6. On the other hand, the coating chamber 2 in which a substrate 10 is placed, is directly connected with a vacuum pump 7 and evacuated at a high rate. Consequently, the partial pressure of the nitrogen in the vicinity of the substrate 10 is kept low. As a result, while the substrate is cleaned by ion bombardment with a noticeable sputtering effect, the generation of molten particles at the cathode of the evaporating source can be suppressed.
The present inventors found that when a subchamber 3 is provided and the pressure of the introduced gas in the vicinity of the evaporating source is kept high, some gases other than nitrogen can have an effect of suppressing the molten particles. For reducing the quantity of molten particles, it is desirable to use at least one type of gas selected from the group consisting of nitrogen, hydrogen, methane, argon, helium, acetylene, and oxygen gases.
When a reactive gas is used, the gas nitrides, oxidizes, or carbonizes the cathode to form compounds with a high melting point on the surface of the cathode, so that the generation of molten particles can be suppressed. The detailed explanation given below uses a nitrided film as an example.
When an inert gas is used, the gas increases the sputtering effect in the cleaning of a substrate. This increase in sputtering effect reduces the cleaning time, thereby reducing the deposited quantity of molten particles.
In the coating process, it is desirable that the gas pressure in the subchamber 3 be controlled to fall within the range of 0.0001 to 10 Pa. If less than 0.0001 Pa, the molten particles cannot be decreased sufficiently. If more than 10 Pa, the partial gas pressure in the vicinity of the substrate is increased excessively, so that the sputtering effect is reduced depending on the type of gas used. Therefore, when the gas pressure fails to fall within the foregoing range, it is difficult to satisfy concurrently the requirements for bonding strength and surface roughness. It is more desirable that the gas pressure be in the range of 0.05 to 1 Pa. The pressure in the subchamber and coating chamber can be measured by a diaphragm gauge.